path: root/usr.sbin/dhcp/server/dhcpd.conf.5

.\"	dhcpd.conf.5
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/isc''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.TH dhcpd.conf 5
.SH NAME
dhcpd.conf - DHCP server configuration file
.SH DESCRIPTION
The dhcpd.conf file contains configuration information for
.IR dhcpd,
the Internet Software Consortium DHCP Server.
.PP
The dhcpd.conf file is a free-form ASCII text file.   It is parsed by
the recursive-descent parser built into dhcpd.   The file may contain
extra tabs and newlines for formatting purposes.  Keywords in the file
are case-insensitive.   Comments may be placed anywhere within the
file (except within quotes).   Comments begin with the # character and
end at the end of the line.
.PP
The file essentially consists of a list of statements.   Statements
fall into two broad categories - parameters and declarations.
.PP
Parameter statements either say how to do something (e.g., how long a
lease to offer), whether to do something (e.g., should dhcpd provide
addresses to unknown clients), or what parameters to provide to the
client (e.g., use gateway 220.177.244.7).
.PP
Declarations are used to describe the topology of the
network, to describe clients on the network, to provide addresses that
can be assigned to clients, or to apply a group of parameters to a
group of declarations.   In any group of parameters and declarations,
all parameters must be specified before any declarations which depend
on those parameters may be specified.
.PP
Declarations about network topology include the
 \fIshared-network\fR and the \fIsubnet\fR
declarations.   If clients on a subnet are to be assigned addresses
dynamically, a \fIrange\fR declaration must appear within the
\fIsubnet\fR declaration.   For clients with statically assigned
addresses, or for installations where only known clients will be
served, each such client must have a \fIhost\fR declaration.   If
parameters are to be applied to a group of declarations which are not
related strictly on a per-subnet basis, the \fIgroup\fR declaration
can be used.
.PP
For every subnet which will be served, and for every subnet
to which the dhcp server is connected, there must be one \fIsubnet\fR
declaration, which tells dhcpd how to recognize that an address is on
that subnet.  A \fIsubnet\fR declaration is required for each subnet
even if no addresses will be dynamically allocated on that subnet.
.PP
Some installations have physical networks on which more than one IP
subnet operates.   For example, if there is a site-wide requirement
that 8-bit subnet masks be used, but a department with a single
physical Ethernet network expands to the point where it has more than
254 nodes, it may be necessary to run two 8-bit subnets on the same
Ethernet until such time as a new physical network can be added.   In
this case, the \fIsubnet\fR declarations for these two networks may be
enclosed in a \fIshared-network\fR declaration.
.PP
Some sites may have departments which have clients on more than one
subnet, but it may be desirable to offer those clients a uniform set
of parameters which are different than what would be offered to
clients from other departments on the same subnet.   For clients which
will be declared explicitly with \fIhost\fR declarations, these
declarations can be enclosed in a \fIgroup\fR declaration along with
the parameters which are common to that department.   For clients
whose addresses will be dynamically assigned, there is currently no
way to group parameter assignments other than by network topology.
.PP
When a client is to be booted, its boot parameters are determined by
first consulting that client's \fIhost\fR declaration (if any), then
consulting the \fIgroup\fR declaration (if any) which enclosed that
\fIhost\fR declaration, then consulting the \fIsubnet\fR declaration
for the subnet on which the client is booting, then consulting the
\fIshared-network\fR declaration (if any) containing that subnet, and
finally consulting the top-level parameters which may be specified
outside of any declaration.
.PP
When dhcpd tries to find a \fIhost\fR declaration for a client, it
first looks for a \fIhost\fR declaration which has a
\fIfixed-address\fR parameter which matches the subnet or shared
network on which the client is booting.   If it doesn't find any such
entry, it then tries to find an entry which has no \fIfixed-address\fR
parameter.   If no such entry is found, then dhcpd acts as if there is
no entry in the dhcpd.conf file for that client, even if there is an
entry for that client on a different subnet or shared network.
.SH EXAMPLES
.PP
A typical dhcpd.conf file will look something like this:
.nf

.I global parameters...

shared-network ISC-BIGGIE {
  \fIshared-network-specific parameters...\fR
  subnet 204.254.239.0 netmask 255.255.255.224 {
    \fIsubnet-specific parameters...\fR
    range 204.254.239.10 204.254.239.30;
  }
  subnet 204.254.239.32 netmask 255.255.255.224 {
    \fIsubnet-specific parameters...\fR
    range 204.254.239.42 204.254.239.62;
  }
}

subnet 204.254.239.64 netmask 255.255.255.224 {
  \fIsubnet-specific parameters...\fR
  range 204.254.239.74 204.254.239.94;
}

group {
  \fIgroup-specific parameters...\fR
  host zappo.test.isc.org {
    \fIhost-specific parameters...\fR
  }
  host beppo.test.isc.org {
    \fIhost-specific parameters...\fR
  }
  host harpo.test.isc.org {
    \fIhost-specific parameters...\fR
  }
}

.ce 1
Figure 1

.fi
.PP
Notice that at the beginning of the file, there's a place
for global parameters.   These might be things like the organization's
domain name, the addresses of the name servers (if they are common to
the entire organization), and so on.   So, for example:
.nf

	option domain-name "isc.org";
	option domain-name-servers ns1.isc.org, ns2.isc.org;

.ce 1
Figure 2
.fi
.PP
As you can see in Figure 2, it's legal to specify host addresses in
parameters as domain names rather than as numeric IP addresses.  If a
given hostname resolves to more than one IP address (for example, if
that host has two Ethernet interfaces), both addresses are supplied to
the client.
.PP
In Figure 1, you can see that both the shared-network statement and
the subnet statements can have parameters.   Let us say that the
shared network \fIISC-BIGGIE\fR supports an entire department -
perhaps the accounting department.   If accounting has its own domain,
then a shared-network-specific parameter might be:
.nf

	option domain-name "accounting.isc.org";
.fi
.PP
All subnet declarations appearing in the shared-network declaration
would then have the domain-name option set to "accounting.isc.org"
instead of just "isc.org".
.PP
The most obvious reason for having subnet-specific parameters as
shown in Figure 1 is that each subnet, of necessity, has its own
router.   So for the first subnet, for example, there should be
something like:
.nf

	option routers 204.254.239.1;
.fi
.PP
Note that the address here is specified numerically.   This is not
required - if you have a different domain name for each interface on
your router, it's perfectly legitimate to use the domain name for that
interface instead of the numeric address.   However, in many cases
there may be only one domain name for all of a router's IP addresses, and
it would not be appropriate to use that name here.
.PP
In Figure 1 there is also a \fIgroup\fR statement, which provides
common parameters for a set of three hosts - zappo, beppo and harpo.
As you can see, these hosts are all in the test.isc.org domain, so it
might make sense for a group-specific parameter to override the domain
name supplied to these hosts:
.nf

	option domain-name "test.isc.org";
.fi
.PP
Also, given the domain they're in, these are probably test machines.
If we wanted to test the DHCP leasing mechanism, we might set the
lease timeout somewhat shorter than the default:

.nf
	max-lease-time 120;
	default-lease-time 120;
.fi
.PP
You may have noticed that while some parameters start with the
\fIoption\fR keyword, some do not.   Parameters starting with the
\fIoption\fR keyword correspond to actual DHCP options, while
parameters that do not start with the option keyword either control
the behaviour of the DHCP server (e.g., how long a lease dhcpd will
give out), or specify client parameters that are not optional in the
DHCP protocol (for example, server-name and filename).
.PP
In Figure 1, each host had \fIhost-specific parameters\fR.   These
could include such things as the \fIhostname\fR option, the name of a
file to upload (the \fIfilename parameter) and the address of the
server from which to upload the file (the \fInext-server\fR
parameter).   In general, any parameter can appear anywhere that
parameters are allowed, and will be applied according to the scope in
which the parameter appears.
.PP
Imagine that you have a site with a lot of NCD X-Terminals.   These
terminals come in a variety of models, and you want to specify the
boot files for each models.   One way to do this would be to have host
declarations for each server and group them by model:
.nf

group {
  filename "Xncd19r";
  next-server ncd-booter;

  host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
  host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
  host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
}

group {
  filename "Xncd19c";
  next-server ncd-booter;

  host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
  host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
}

group {
  filename "XncdHMX";
  next-server ncd-booter;

  host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
  host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
  host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }
}
.fi
.SH REFERENCE: DECLARATIONS
.PP
.B The
.I shared-network
.B statement
.PP
.nf
 \fBshared-network\fR \fIname\fR \fB{\fR
   [ \fIparameters\fR ]
   [ \fIdeclarations\fR ]
 \fB}\fR
.fi
.PP
The \fIshared-network\fR statement is used to inform the DHCP server
that some IP subnets actually share the same physical network.  Any
subnets in a shared network should be declared within a
\fIshared-network\fR statement.  Parameters specified in the
\fIshared-network\fR statement will be used when booting clients on
those subnets unless parameters provided at the subnet or host level
override them.  If any subnet in a shared network has addresses
available for dynamic allocation, those addresses are collected into a
common pool for that shared network and assigned to clients as needed.
There is no way to distinguish on which subnet of a shared network a
client should boot.
.PP
.I Name
should be the name of the shared network.   This name is used when
printing debugging messages, so it should be descriptive for the
shared network.   The name may have the syntax of a valid domain name
(although it will never be used as such), or it may be any arbitrary
name, enclosed in quotes.
.PP
.B The
.I subnet
.B statement
.PP
.nf
 \fBsubnet\fR \fIsubnet-number\fR \fBnetmask\fR \fInetmask\fR \fB{\fR
   [ \fIparameters\fR ]
   [ \fIdeclarations\fR ]
 \fB}\fR
.fi
.PP
The \fIsubnet\fR statement is used to provide dhcpd with enough
information to tell whether or not an IP address is on that subnet.
It may also be used to provide subnet-specific parameters and to
specify what addresses may be dynamically allocated to clients booting
on that subnet.   Such addresses are specified using the \fIrange\fR
declaration.
.PP
The
.I subnet-number
should be an IP address or domain name which resolves to the subnet
number of the subnet being described.   The
.I netmask
should be an IP address or domain name which resolves to the subnet mask
of the subnet being described.   The subnet number, together with the
netmask, are sufficient to determine whether any given IP address is
on the specified subnet.
.PP
Although a netmask must be given with every subnet declaration, it is
recommended that if there is any variance in subnet masks at a site, a
subnet-mask option statement be used in each subnet declaration to set
the desired subnet mask, since any subnet-mask option statement will
override the subnet mask declared in the subnet statement.
.PP
.B The
.I range
.B statement
.PP
.nf
 \fBrange\fR [ \fBdynamic-bootp\fR ] \fIlow-address\fR [ \fIhigh-address\fR]\fB;\fR
.fi
.PP
For any subnet on which addresses will be assigned dynamically, there
must be at least one \fIrange\fR statement.   The range statement
gives the lowest and highest IP addresses in a range.   All IP
addresses in the range should be in the subnet in which the
\fIrange\fR statement is declared.   The \fIdynamic-bootp\fR flag may
be specified if addresses in the specified range may be dynamically
assigned to BOOTP clients as well as DHCP clients.   When specifying a
single address, \fIhigh-address\fR can be omitted.
.PP
.B The
.I host
.B statement
.PP
.nf
 \fBhost\fR \fIhostname\fR {
   [ \fIparameters\fR ]
   [ \fIdeclarations\fR ]
 \fB}\fR
.fi
.PP
There must be at least one
.B host
statement for every BOOTP client that is to be served.
.B host
statements may also be specified for DHCP clients, although this is
not required unless booting is only enabled for known hosts.
.PP
If it is desirable to be able to boot a DHCP or BOOTP
client on more than one subnet with fixed addresses, more than one
address may be specified in the
.I fixed-address
parameter, or more than one
.B host
statement may be specified.
.PP
If client-specific boot parameters must change based on the network
to which the client is attached, then multiple
.B host
statements should
be used.
.PP
If a client is to be booted using a fixed address if it's
possible, but should be allocated a dynamic address otherwise, then a
.B host
statement must be specified without a
.B fixed-address
clause.
.I hostname
should be a name identifying the host.  If a \fIhostname\fR option is
not specified for the host, \fIhostname\fR is used.
.PP
\fIHost\fR declarations are matched to actual DHCP or BOOTP clients
by matching the \fRdhcp-client-identifier\fR option specified in the
\fIhost\fR declaration to the one supplied by the client, or, if the
\fIhost\fR declaration or the client does not provide a
\fRdhcp-client-identifier\fR option, by matching the \fIhardware\fR
parameter in the \fIhost\fR declaration to the network hardware
address supplied by the client.   BOOTP clients do not normally
provide a \fIdhcp-client-identifier\fR, so the hardware address must
be used for all clients that may boot using the BOOTP protocol.
.PP
.B The
.I group
.B statement
.PP
.nf
 \fBgroup\fR {
   [ \fIparameters\fR ]
   [ \fIdeclarations\fR ]
 \fB}\fR
.fi
.PP
The group statement is used simply to apply one or more parameters to
a group of declarations.   It can be used to group hosts, shared
networks, subnets, or even other groups.
.SH REFERENCE: ALLOW and DENY
.PP
The
.I allow
and
.I deny
statements can be used to control the behaviour of dhcpd to various
sorts of requests.
.PP
.PP
.B The
.I unknown-clients
.B keyword
.PP
 \fBallow unknown-clients;\fR
 \fBdeny unknown-clients;\fR
.PP
The \fBunknown-clients\fR flag is used to tell dhcpd whether
or not to dynamically assign addresses to unknown clients.   Dynamic
address assignment to unknown clients is \fBallow\fRed by default.
.PP
.B The
.I bootp
.B keyword
.PP
 \fBallow bootp;\fR
 \fBdeny bootp;\fR
.PP
The \fBbootp\fR flag is used to tell dhcpd whether
or not to respond to bootp queries.  Bootp queries are \fBallow\fRed
by default.
.PP
.B The
.I booting
.B keyword
.PP
 \fBallow booting;\fR
 \fBdeny booting;\fR
.PP
The \fBbooting\fR flag is used to tell dhcpd whether or not to respond
to queries from a particular client.  This keyword only has meaning
when it appears in a host declaration.   By default, booting is
\fBallow\fRed, but if it is disabled for a particular client, then
that client will not be able to get and address from the DHCP server.
.SH REFERENCE: PARAMETERS
.PP
.B The
.I default-lease-time
.B statement
.PP
 \fBdefault-lease-time\fR \fItime\fR\fB;\fR
.PP
.I Time
should be the length in seconds that will be assigned to a lease if
the client requesting the lease does not ask for a specific expiration
time.
.PP
.B The
.I max-lease-time
.B statement
.PP
 \fBmax-lease-time\fR \fItime\fR\fB;\fR
.PP
.I Time
should be the maximum length in seconds that will be assigned to a
lease if the client requesting the lease asks for a specific
expiration time.
.PP
.B The
.I hardware
.B statement
.PP
 \fBhardware\fR \fIhardware-type\fR \fIhardware-address\fR\fB;\fR
.PP
In order for a BOOTP client to be recognized, its network hardware
address must be declared using a \fIhardware\fR clause in the
.I host
statement.
.I hardware-type
must be the name of a physical hardware interface type.   Currently,
only the
.B Ethernet
type is recognized, although support for
.B token-ring
and
.B fddi
hardware types would also be desirable.
The
.I hardware-address
should be a set of hexadecimal octets (numbers from 0 through ff)
separated by colons.   The \fIhardwarefR statement may also be used
for DHCP clients.
.PP
.B The
.I filename
.B statement
.PP
 \fBfilename\fR \fB"\fR\fIfilename\fR\fB";\fR
.PP
The \fIfilename\fR statement can be used to specify the name of the
initial boot file which is to be loaded by a client.  The
.I filename
should be a filename recognizable to whatever file transfer protocol
the client can be expected to use to load the file.
.PP
.B The
.I server-name
.B statement
.PP
 \fBserver-name\fR \fB"\fR\fIname\fR\fB";\fR
.PP
The \fIserver-name\fR statement can be used to inform the client of
the name of the server from which it is booting.   \fIName\fR should
be the name that will be provided to the client.
.PP
.B The
.I next-server
.B statement
.PP
 \fBnext-server\fR \fIserver-name\fR\fB;\fR
.PP
The \fInext-server\fR statement is used to specify the host address of
the server from which the initial boot file (specified in the
\fIfilename\fR statement) is to be loaded.   \fIServer-name\fR should
be a numeric IP address or a domain name.   If no \fInext-server\fR
parameter applies to a given client, the DHCP server's IP address is
used.
.PP
.B The
.I fixed-address
.B statement
.PP
 \fBfixed-address\fR \fIaddress\fR [\fB,\fR \fIaddress\fR ... ]\fB;\fR
.PP
The \fIfixed-address\fR statement is used to assign one or more fixed
IP addresses to a client.  It should only appear in a \fIhost\fR
declaration.  If more than one address is supplied, then when the
client boots, it will be assigned the address which corresponds to the
network on which it is booting.  If none of the addresses in the
\fIfixed-address\fR statement are on the network on which the client
is booting, that client will not match the \fIhost\fR declaration
containing that \fIfixed-address\fR statement.  Each \fIaddress\fR
should be either an IP address or a domain name which resolves to one
or more IP addresses.
.PP
.B The
.I dynamic-bootp-lease-cutoff
.B statement
.PP
 \fBdynamic-bootp-lease-cutoff\fR \fIdate\fR\fB;\fR
.PP
The \fIdynamic-bootp-lease-cutoff\fR statement sets the ending time
for all leases assigned dynamically to BOOTP clients.  Because BOOTP
clients do not have any way of renewing leases, and don't know that
their leases could expire, by default dhcpd assigns infinite leases
to all BOOTP clients.  However, it may make sense in some situations
to set a cutoff date for all BOOTP leases - for example, the end of a
school term, or the time at night when a facility is closed and all
machines are required to be powered off.
.PP
.I Date
should be the date on which all assigned BOOTP leases will end.  The
date is specified in the form:
.PP
.ce 1
W YYYY/MM/DD HH:MM:SS
.PP
W is the day of the week expressed as a number
from zero (Sunday) to six (Saturday).  YYYY is the year, including the
century.  MM is the month expressed as a number from 1 to 12.  DD is
the day of the month, counting from 1.  HH is the hour, from zero to
23.  MM is the minute and SS is the second.  The time is always in
Greenwich Mean Time (GMT), not local time.
.PP
.B The
.I dynamic-bootp-lease-length
.B statement
.PP
 \fBdynamic-bootp-lease-length\fR \fIlength\fR\fB;\fR
.PP
The \fIdynamic-bootp-lease-length\fR statement is used to set the
length of leases dynamically assigned to BOOTP clients.   At some
sites, it may be possible to assume that a lease is no longer in
use if its holder has not used BOOTP or DHCP to get its address within
a certain time period.   The period is specified in \fIlength\fR as a
number of seconds.   If a client reboots using BOOTP during the
timeout period, the lease duration is reset to \fIlength\fR, so a
BOOTP client that boots frequently enough will never lose its lease.
Needless to say, this parameter should be adjusted with extreme
caution.
.PP
.B The
.I get-lease-hostnames
.B statement
.PP
 \fBget-lease-hostnames\fR \fIflag\fR\fB;\fR
.PP
The \fIget-lease-hostnames\fR statement is used to tell dhcpd whether
or not to look up the domain name corresponding to the IP address of
each address in the lease pool and use that address for the DHCP
\fIhostname\fR option.  If \fIflag\fR is true, then this lookup is
done for all addresses in the current scope.   By default, or if
\fIflag\fR is false, no lookups are done.
.PP
.B The
.I use-host-decl-names
.B statement
.PP
 \fBuse-host-decl-names\fR \fIflag\fR\fB;\fR
.PP
If the \fIuse-host-decl-names\fR parameter is true in a given scope,
then for every host declaration within that scope, the name provided
for the host declaration will be supplied to the client as its
hostname.   So, for example,
.PP
.nf
    group {
      use-host-decl-names on;

      host joe {
	hardware ethernet 08:00:2b:4c:29:32;
	fixed-address joe.fugue.com;
      }
    }

is equivalent to

      host joe {
	hardware ethernet 08:00:2b:4c:29:32;
	fixed-address joe.fugue.com;
        option host-name "joe";
      }
.fi
.PP
An \fIoption host-name\fR statement within a host declaration will
override the use of the name in the host declaration.
.PP
.B The
.I server-identifier
.B statement
.PP
 \fBserver-identifier \fIhostname\fR\fB;\fR
.PP
The server-identifier statement is now obsolete and is ignored by
the DHCP server.
.SH REFERENCE: OPTION STATEMENTS
.PP
DHCP option statements are documented in the
.B dhcp-options(5)
manual page.
.SH SEE ALSO
dhcpd.conf(5), dhcpd.leases(5), RFC2132, RFC2131.
.SH AUTHOR
.B dhcpd(8)
was written by Ted Lemon <mellon@vix.com>
under a contract with Vixie Labs.   Funding
for this project was provided by the Internet Software Corporation.
Information about the Internet Software Consortium can be found at
.B http://www.isc.org/isc.